My Darwin Day Contribution or On a Mechanism of Speciation

How do new species originate? This is a question I am not going to ultimately answer here, but I am interested in this issue and have had some thoughts about it. Now the first issue that needs to be dealt with is "what is a species?" Since many people have been fighting with that question for centuries without coming up with a clear precise definition that applies to all organisms, I am not about to answer it here. So, I will limit our concept of species to sexually reproducing organisms. This allows us to define a species as a group of organisms that can interbreed and give rise to fertile offspring. This allows us to separate horses and donkeys into separate species, since mules are viable but not fertile.
OK, so we have defined a species. Now what? Well in general terms there are four ways speciation can occur: allopatric, parapatric, peripatric, and sympatric speciation. The first three of these modes of speciation involve the spatial separation of two populations of a species in various ways. Once separate, the two populations then evolve independently ultimately becoming distinct species. Its the last mode of speciation, sympatric, I am interested in here.

In sympatric speciation the two populations are not separate but occupy the same niche. There are several models describing how sympatric speciation can occur, including sexual selection models and polyploidization. However, I want to discuss a third model which is based on an initial genetic isolation. The paper that really got me thinking about it was one we discussed in my course from PLoS Genetics. Well this paper and the fact that I have never been happy with the primacy of allopatric speciation (a post for another time).

First it is helpful to think about the ultimate output of the genetic material (aka genes), which is proteins. In many, if not most, cases, proteins do not act as individual molecules, doing their job, and moving on. In fact, most proteins act in complexes with other proteins. What this means is that the proteins must physically interact with each other, and if they don't, the process they are involved in fails. Second, there is a lot of variation out there in the world. Most of this variation is neutral, neither good nor bad, it just is. (Bad variation generally disappears quickly; good variation generally is fixed quickly.) These are our assumptions both of which are well documented, read some textbooks/journal articles for support of my first assumption (proteins act in complexes); go to the mall and people watch for support of my second assumption (variation is out there).

OK, lets take a hypothetical example. In the species of blue-butted gnus, two proteins (A and B) interact to make the blue pigment in said gnus butts. In the first couple of blue-butted gnus we studied, we saw that the gnus had alleles A1 and B1 of these proteins and of course their butts were blue. However, we looked at several other gnus and identified alleles A2 and B2 of these proteins. Upon closer examination, we find lots of A1B1 gnus and a few A1B2 and A2B1 gnus, but no A2B2 gnus. That seems odd, so as junior scientist-heroes, we grab an A1B2 animal and mate it to an A2B1 animal and find out that low and behold we can get an A2B2 animal!!!11!1 But HOLY SHIT, it has a green butt! Did we just make a new species of green-butted gnus? Sadly, we find that gnus, regardless of butt color only want to mate with blue-butted gnus with blue butts. So, no gnus will mate with a green-butted gnu, which as you know is not good news for the green-butted gnu. Now the A2 and B2 alleles do not matter at all, except when combined together, so this has the effect that gnus with the A2 or B2 alleles tend to become isolated, the A2 gnus tend to be at the left hand side of the field over by the swamp, whereas the B2 gnus are found more often at the other side of the field near the woods. What we've done is begun to genetically isolate populations. So although all blue-butted gnus can mate, green-butted gnus are immediately removed from the population. Of course random drift can lead to the complete loss of either A2 or B2, which would end this discussion. However, additional random mutations can and will occur and genetic drift now has a mechanism that can further genetically isolate these gnus into distinct, but spatially overlapping populations. Give me another 40,000 years and a glass of wine and Ill give you two species of gnus: the swampland blue-butted gnu and the woodland blue-butted gnu.

That was hypothetical example (my wife says "lame" is a better word than hypothetical), the paper is a real life example. Saccharomyces cerevisiae (beer/bakers yeast) has been isolated from many sources over the last hundred years and studied in detail. S. cerevisiae is great because it is stable as a haploid or diploid and comes in two mating types and you can mate them in the lab no problem (which is why S. cerevisiae is THE eukaryotic genetic system. One thing that was noted early in studies with S. cerevisiae is that some strains do not mate to give rise to viable progeny. So, strains A and B do not mate well. However, strain C can mate with either A or B just fine. So what gives? Well, the read the paper, but the short story is that there are alleles of genes encoding proteins in an essential complex. When these alleles are in the right combinations the proteins are incompatible and make a defective complex, which equals death, or at least a lack of life.

I think that this model, genetic isolation, may be a common mechanism of separating populations, which can lead to speciation. So why are the other three mechanisms so widely taught (particularly allopatric speciation)? Well its simple to observe and study: find an island and see how the species are similar/different from the mainland. Its also conceptually a little easier, a big giant mountain range comes up splitting a species into two distinct populations, they can and will vary independently. But just because something is easier to observe, doesnt mean its the most important just the most studied (take natural selection vs genetic drift).

Happy Darwin Day everyone!!!

Image credit

(Incompatibilities involving yeast mismatch repair genes: a role for genetic modifiers and implications for disease penetrance and variation in genomic mutation rates. Demogines A, Wong A, Aquadro C, Alani E. PLoS Genet. 2008 Jun 20;4(6):e1000103.)